Over the past few years, the complexity and capabilities of integrated circuits (IC's) have grown while their dimensions continued to shrink. The rapid advances in semiconductor technology nearly doubles the number of circuit functions per semiconductor chip per year. In order to keep pace with the very large scale integration technology, the semiconductor chips require higher pin out packages having small sizes thereby achieving optimal electronic performance and increased circuit board densities. Because of the foregoing, the well known dual in-line packages (DIP's) are gradually being replaced by surface-mounted chip carrier packages.
Coincident with the higher packing density of surface-mounted components is the corresponding reduction in solder joint size and spacing on the printed wiring board (PWB), as well as the increase in number of solder joints on each PWB assembly. Several methods of solder deposition are described in a paper by D. Schoenthaler entitled "Soldering Circuit Assemblies in the 1980's" published in Proceedings of Printed Circuit World Convention II, Munich, West Germany, Vol. 1, pages 131-140, June 9-12, 1981. When using screen or stencil printing of solder on a PWB, the volume of solder deposited on the PWB must be sufficient to prevent joint opens due to poor component lead planarity, PWB warpage, inaccurate component placement and misregistration of the solder mask with respect to the bond sites of the PWB. On the other hand, in view of the reduced spacing between neighboring solder joints on the PWB, the quantity of solder deposited should not be excessive to avoid an electrical short between two adjacent solder joints.
As mentioned in the above reference by D. Schoenthaler, an alternative method for depositing solder paste on PWB's is a technique using a multipoint dispensing tool comprising a hollow chamber having a multiorifice nozzle attached at one end thereof. The nozzle orifices are spaced to match the PWB footprint pattern and the corresponding chip carrier lead geometry. Applicants observed that the implementation of multipoint dispensing techniques in a manufacturing environment encountered various problems affecting the reliability and reproducibility of the solder joints. Some of these problems include the poor control of the volume and geometry of the dispensed solder paste, clogging of the orifices of the nozzle, variations in the paste viscosity, and difficulty in achieving uniform paste distribution among the orifices due to paste crust and/or void formations within the dispensing tool.
Therefore, there exists a need for an improved solder paste multipoint dispensing technique capable of being reliably and reproducibly implemented in a semiconductor manufacturing environment.